A Re-examination of the Portevin-Le Chatelier Effect in Alloy 718 in Connection with Oxidation-Assisted Intergranular Cr
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ALLOY 718 is a widely used superalloy for high performance applications because of its excellent mechanical properties over a wide range of temperatures. Nevertheless, alloy 718 is also well known to be sensitive to intergranular stress crack growth.[1,2] This sensitivity to stress corrosion cracking has been observed for various simulated in service conditions for different types of applications (including nuclear power plants and aeronautical industries).[3,4] It was shown that the damaging mechanisms encountered in pressurized water reactors [623 K (350 C) in water] are similar to the ones found at higher temperature under air [923 K (650 C) in air].[5] Alloy 718 also exhibits dynamic strain aging (DSA) phenomenon[6–10] related to dynamic interactions between mobile solute atoms and the moving dislocations. Within a limited domain of the temperature vs strain rate plane, this DSA results in plastic instabilities called Portevin-Le Chatelier effect, which may often be revealed by the occurrence of serrations on the tensile curve. Despite the fact that PLC domain is obviously a sub-domain of DSA phenomenon, for sake of simplicity we will denote throughout this paper ‘‘PLC’’ the temperature and strain rate ranges in which the plastic instabilities occur and ‘‘DSA’’ the BERTRAND MAX, Ph.D. Researcher, BERNARD VIGUIER and ERIC ANDIREU, Professors, and JEAN MARC CLOUE, Senior Researcher, are with the CIRIMAT, ENSIACET/INP, Universite´ de Toulouse, 4 Alle´e Emile Monso, BP 44362, 31030 Toulouse Cedex 4, France. Contact e-mail: [email protected] Manuscript submitted January 17, 2014. Article published online August 14, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
domain without instabilities. Fournier et al.,[11] while studying the effect of oxidation on the mechanical behavior of alloy 718 for temperatures ranging from 673 K to 873 K (400 C to 600 C) and at a strain rate of 5 9 107 s1, showed that the occurrence of PLC phenomenon during tensile test systematically resulted in the absence of brittle area on the fracture surface of the samples. By contrast when the stress-strain curves are smooth intergranular cracking areas were always observed on the fracture surface. This finding was further confirmed by Garat et al.[12] for tensile tests in air at temperatures higher than 673 K (400 C) and over a wide range of strain rates (105 to 103 s1). These authors showed that the fracture mode domain (intergranular or not) and the deformation mode domain (smooth or serrated) superimpose in the strain rate vs temperature plane. The first explanations for such an observation mainly considered the mechanical aspect of the PLC phenomenon. However, the phenomena like the DSA/PLC deformation modes obviously include a ‘‘chemical’’ aspect. For instance Ter-Ovanessian et al.[13] evidenced the displacement of PLC domain and the rupture mode domain in the temperature-strain rate plane caused by the diminution of interstitial content of the same heat of alloy 718. These experimental results lead to consider the poss
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